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Unformatted text preview: s an output signal of 1 if any of the input signals is also
1.
Two or more switches connected in parallel behave as an OR gate. It can be seen from
Figure 6.20 that the input current will reach the output point when any one of the two
switches are in the on(l) state. There will be no output only when both the switches»(A
andJB) are in the off(0) state.
The truth table and the block diagram symbol for an OR gate for two input signals are
shown in Figure 6.21. Observe that an output of 1 is obtained when any of the input
signals is 1. Output is 0 only when both the inputs are 0.
Inputs
Output
A
0
0
1
1 8
0
1
0
1 C = A +B
0
1
1
1 Figure 6.21. Block diagram symbol and truth table for an OR gate.
Just as the + and ■ operations could be extended to several variables using the associative
law, AND gates and OR gates can have more than two inputs. Figure 6.22 shows three
input AND and OR gates and the table of all input combinations for each. As might be
hoped, the output of the AND gate with inputs A, B and C is a 1 only if A and B and C
are 1, i.e., when all three of the inputs are 1, so that we write the output as A • B • C.
Similarly, the OR gate with inputs A, B, and C has a 1 output if A or B or C is a 1, so that
we can write A + B + C for its output.
The above argument can be extended. A fourinput AND gate has a 1 output only when
all four inputs are 1, and a fourinput OR gate has a 1 output when any of its inputs is a 1.
ABC D = ABC
Inputs Output A
B
0
0
0
0
0
1
0
1
1
0
1
0
1
1
1
1
(a) Three input AND gate. C
0
1
0
1
0
1
0
1 D=A+B+C
Inputs
A
B
0
0
0
0
0
1
0
1
1
0
1
0
1
1
1
1
(b) Three input OR gate. D = ABC
0
0
0
0
0
0
0
1 Output
c
0
1
0
1
0
1
0
1 D=A+B+C
0
1
1
1
1
1
1
1 Figure 6.22. Block diagram symbol and truth table for
(a) Three input AND gate
(b) Three input OR gate.
NOT Gate
A NOT gate is the physical realization of the complementation operation. That is, it is an
electronic circuit that generates an output signal, which is the reverse of the input signal.
A NOT gate is also...
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This document was uploaded on 04/07/2014.
 Spring '14

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